I would also like to thank jsmorley for creating RainRez and giving us such a great tool. Also thanks to ouned and hello2u for their additions!

There is some issue with RainRez looping the Layout change when it detects some resolution changes. I haven't really been able to determine what triggers this looping to try and find a way to prevent it. It seems to happen when connecting/disconnecting an additional display.

I've made a few tweaks to their source in case it can help anyone else. I'm not a coder, just took what they had already provided (or maybe I didn't understand enough to get it to do what I wanted?) and added some settings you can set in RainRez.cfg:

Source:
(Credit to jsmorley, hello2u, and ouned as I just took what I saw in their work and mashed it all into one with some minor tweaks.)

[RainRez]
RainmeterPath=C:\Utils\Rainmeter
ResolutionMethod=Native
; ^-- Native / Virtual
; Native/Virtual will determine how the resolution
; is calculated, using either the Native resolution, or the Virtual resolution.
LayoutMethod=Aspect
; Resolution/Aspect will set the layout selection method
; using either the detected resolutions (based on ResolutionMethod),
; or by determining if the aspect ratio is Wide or Standard, then load the
; layout as defined in the .CFG for Wide/Standard aspect settings respectively,
; or load the resolution layout. If a layout for the resolution is not defined
; in the .CFG, we fall back to use the defined Default layout.
; (if the numberator divided by denominator > 1.3334 then
; it's considered a wide aspect ratio)
Default=4.3
Battery=Battery
Wide=16.9
Standard=4.3
10240x4320=16.9
8192x4608=16.9
5120x2880=16.9
5120x2160=16.9
3440x1440=16.9
3200x2400=4.3
3000x2000=4.3
2736x1824=4.3
2560x1600=4.3
2048x1152=16.9
1920x1200=4.3
1920x1080=16.9
1680x1050=16.9
1440x900=16.9
1280x1024=4.3
1280x800=16.9
1280x768=16.9
1024x768=4.3
800x600=4.3
640x480=640x480

Edit:
Added compiled files for convenience. Note that neither myself nor the other authors will be held accountable for issues, support, etc. that you may encounter. You can always (and as a best practice, should) compile the source yourselves. Blah blah.

You do not have the required permissions to view the files attached to this post.